1. Field of the Invention
The present invention relates to a novel molding die for an optical recording disk. Particularly, the invention relates to a technique for preventing or restraining a wedge-shaped deformation from being formed at an outer peripheral portion of an optical recording disk.
2. Description of the Related Art
In general, an optical recording disk, such as a CD, CD-R, or MO, is formed by forming a substrate through injection molding of a thermosetting resin and then, by carrying out processing of formation of a reflecting layer and the like.
When the optical recording disk (substrate) xe2x80x9caxe2x80x9d is injection molded, there is a problem that a wedge-like deformation xe2x80x9cbxe2x80x9d is formed at an outer peripheral portion (see a solid line of FIG. 2). It appears that the reason why such a deformation xe2x80x9cbxe2x80x9d is formed is as follows:
When a resin is cooled in a cavity of a molding die, upper and lower surfaces (main surfaces) and inner and outer peripheral surfaces are first cooled, and a shell is formed at the outside. Thereafter, as the inside is cooled, contraction continues, and plate thickness is decreased at portions except for the outer peripheral portion. At the outer peripheral portion, the previously formed shell resists in the upper and lower directions, so that the portion does not contract, or even if it contracts, the contraction is merely slight. Thus, when its section is seen, only the outer periphery is widened and comes to have a shape like a wedge (solid line in FIG. 2).
There is a problem that when the foregoing wedge-shaped deformation is formed on the optical recording disk, a recording region can not be widened to a position close to the outer periphery.
An object of the present invention is therefore to prevent or restrain a wedge-shaped deformation from being formed at an outer peripheral portion of an optical recording disk.
In order to achieve the above object, in a molding die for an optical recording disk according to the invention, a cooling rate of an outer peripheral member coming in contact with a portion of the optical recording disk which becomes an outer peripheral surface is made slower than a cooling rate of an opposite member coming in contact with a portion of the optical recording disk which becomes a main surface.
Thus, in the molding die for the optical recording disk according to the invention, since a cooling rate of a portion of an injected molten resin coming in contact with the outer peripheral member, that is, a portion which becomes the outer peripheral surface of the optical recording disk becomes slower than a cooling rate of a portion coming in contact with the opposite member, formation of a shell at the outer peripheral portion becomes slower than that at the main surface portion. Thus, a wedge-shaped deformation is not formed or even if the deformation is formed, it becomes small.